The present invention relates to a one-shot process for the production of elastic molded articles having a closed surface layer by the reaction injection molding technique.
The production of molded articles having a closed surface layer by the isocyanate polyaddition process is known. Such articles may be made, for example, by introducing a reactive (optionally foamable) mixture based on compounds containing several reactive hydrogen atoms and polyisocyanates into a mold (see e.g. German Auslegeschrift No. 1,196,864). The compounds with reactive hydrogen atoms typically used are polyethers containing hydroxyl groups. Examples of suitable polyisocyanates include 2,4- and 2,6-tolylene diisocyanate, their isometric mixtures and polyphenylpolymethylene polyisocyanates obtained by anilineformaldehyde condensation followed by phosgenation. Water and/or fluorinated hydrocarbons may be used as blowing agents. Catalysts known to those in the art to be useful for the production of polyurethanes are generally also used.
Depending upon the starting components (and chain lengthening agents such as glycols or diamines if used) used, it is possible to obtain both elastic and rigid products and variations between these extremes by this procedure. For molded articles that must withstand heavy wear, it is customary to use slightly branched raw materials because such materials yield a product having elastomer-like characteristics. Molded articles of this kind have been produced on a technical scale (e.g. as shoe soles in the shoe manufacturing industry). Large molded articles of this type are used in the automobile industry. These car body parts are generally manufactured by the so called reaction injection molding process (RIM process). This molding process employs a special technique of filling a mold in which the highly reactive, liquid starting components are mixed in so called positively controlled or force operated mixing heads and then injected into the mold at high speed by high pressure dosing apparatus operating at a high output rate.
German Offenlegungsschrift No. 2,622,951 (U.S. Pat. No. 4,218,543) describes how even extremely reactive systems such as one-shot mixtures of diisocyanates or polyisocyanates based on 4,4'-diisocyanato diphenyl methane which are liquid at room temperature, active aromatic polyamines, relatively high molecular weight polyhydroxyl compounds containing primary hydroxyl groups and powerful catalysts with cream times of even less than 1 second can be processed by this method. In such systems, the transition from the liquid to the solid phase is virtually instantaneous, so that the liquid reaction mixture solidifies on the walls of the mold. One advantage of this system is that removal from geometrically simple plate shaped molds does not require the use of external mold release agents. Where such mold release agents are required for mass production, the mold release agents must be applied at regular intervals and during the time of their application, the mold is unavailable for production. Fine grooves etched into the mold gradually become covered with residues of mold release agent. Removal of these firmly adhering residues from the molds (which are in many cases highly contoured) can only be achieved with great difficulty. Moreover, the molded articles are covered with a thin film of mold release agent to which lacquer systems will not adhere. The molded articles must therefore be rubbed down before they are lacquered, or washed with solvent to ensure firm adherence of the lacquer to the plastic surface.
If the mold release agents known from the patent literature, such as those which are quite suitable for the production of self-releasing foam products based on polyurethanes (see e.g. British Pat. No. 1,365,215, U.S. Pat. Nos. 3,726,952; 4,033,912; 4,024,090; 4,058,492 and 4,076,695, and German Offenlegungsschriften Nos. 2,427,273; 2,431,968; 2,307,589 and 2,319,648) are used with the special reaction mixtures described in German Offenlegungsschrift No. 2,622,951, those release agents generally improve the ease of separation of the elastomers produced by the reaction injection molding technique slightly, if at all. Moreover, inclusion of mold release agents containing acid groups (in particular carboxyl groups) in the formulations of German Offenlegungsschrift No. 2,622,951 interferes with catalysis of the highly reactive systems so that the molded products obtained have no initial strength.